Chuck for edge bevel removal and method for centering a wafer prior to edge bevel removal

ABSTRACT

A chuck useful for supporting a wafer during an edge bevel removal (EBR) process comprises a rotatable center hub having a plurality of support arms extending outwardly from the rotatable center hub, support pins on ends of the support arms, gas passages extending through upper surfaces of the support pins, and gas conduits in the support arms, the gas conduits configured to supply gas to the gas passages or apply a vacuum to the gas passages. The support arms can include alignment cams which are rotatable from an outer non-alignment position away from a periphery of the wafer to an inner alignment position at which the wafer is centered. To supply gas or apply a vacuum force to the gas outlets in the support pins, the rotatable center hub can have a gas inlet and a plurality of gas delivery ports in fluid communication with the gas delivery conduits in the support arms. Gas can be supplied to the gas outlets by a source of pressurized gas connected to the gas inlet and suction can be applied to the gas outlets by a vacuum source connected to the gas inlet. During centering, the wafer is floated on a gas cushion which reduces wear of the support pins.

CLAIM OF PRIORITY

This application is a divisional of and claims the benefit of priorityunder 35 U.S.C. § 120 to U.S. patent application Ser. No. 15/092,222,filed on Apr. 6, 2016, which is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

This invention relates to chuck technology for supporting semiconductorwafers. More particularly, it pertains to a method of centering a waferprior to an edge bevel removal (EBR) process wherein liquid etchantsremove unwanted metal for the outer edge of a wafer.

BACKGROUND

In a typical copper Damascene process, the formation of the desiredconductive routes generally begins with a thin physical vapor deposition(PVD) of the metal, followed by a thicker electrofill layer (which isformed by electroplating). The PVD process is typically sputtering. Inorder to maximize the size of the wafer's useable area (sometimesreferred to herein as the “active surface region”) and, thereby,maximize the number of integrated circuits produced per wafer), theelectrofilled metal must be deposited to very near the edge of thesemiconductor wafer. Thus, it is necessary to allow physical vapordeposition of the metal over the entire front side of the wafer. As abyproduct of this process step, PVD metal typically coats the front edgearea outside the active circuit region, as well as the side edge, and tosome degree, the backside.

Electrofill of the metal is much easier to control, since theelectroplating apparatus can be designed to exclude the electroplatingsolution from undesired areas such as the edge and backside of thewafer. One example of plating apparatus that constrains electroplatingsolution to the wafer active surface is the SABRE™ clamshellelectroplating apparatus available from Novellus Systems, Inc. of SanJose, Calif. and described in U.S. Pat. No. 6,156,167 “ClamshellApparatus For Electrochemically Treating Semiconductor Wafers,” by E.Patton et al., and filed Nov. 13, 1997, which is herein incorporated byreference in its entirety.

The PVD metal remaining on the wafer edge after electrofill isundesirable for various reasons. For example, the PVD metal on the waferedge is not suitable for subsequent depositions and tends to flake offgenerating undesirable particles. By contrast the PVD metal on theactive interior region of the wafer is simply covered with thick andeven electrofill metal and planarized by CMP down to the dielectric.This flat surface, which is mostly dielectric, is covered with a barrierlayer, such as silicon nitride or silicon carbide, that both adhereswell to the dielectric and aids in the adhesion of subsequent layers.Unfortunately, the barrier layer, which like the residual PVD metallayers deposits over the wafer edge area, is often thin and uneven andtherefore may allow migration of the metal into the dielectric. Thisproblem is especially important when the metal is copper.

To address these problems, semiconductor equipment may have to allowetching of the unwanted residual metal layers. Various difficulties willbe encountered in designing a suitable etching system. For example, oneof the main constraints of edge bevel removal (EBR) is a relatively longprocessing time. Smaller node technology allows significant reduction ofplating time for thin films. In order to realize the throughput gain, itis highly desirable to reduce duration of all non-plating processes,such as EBR. Additional problems include controlling the etching areaduring the EBR process. It is desirable to minimize losses ofelectro-filled metal in the active area of the wafer while completelyremoving the surrounding bevel (i.e. to reduce “taper width” of thedeposited metal). Overall, improved edge bevel removal methods andapparatuses are desired. Commonly assigned U.S. Pat. No. 8,419,964“Apparatus And Method For Edge Bevel Removal Of Copper From SiliconWafers,” by K. Ganesan et al., and filed Aug. 27, 2008, which is hereinincorporated by reference, discloses an apparatus for performing EBRwherein a wafer is supported on a rotatable chuck having support pinsand alignment pins. A suitable chuck is described in commonly assignedU.S. Pat. No. 6,537,416 “Wafer Chuck For Use In Edge Bevel Removal OfCopper From Silicon Wafers,” by S. Mayer et al., and filed Apr. 25,2000, which is herein incorporated by reference. During wafer centering,a wafer slides on rubber support pins which can lead to wear andparticle problems. It would be desirable to extend the wear of thesupport pins and reduce particle generation during wafer centering.

SUMMARY OF THE INVENTION

According to an embodiment, a method of centering a semiconductor waferprior to an edge bevel removal (EBR) process comprises: (a) transferringa wafer above a rotatable chuck having at least three support arms withsupport pins at outer portions of the support arms, (b) lowering thewafer onto the support pins, (c) supplying pressurized gas to gaspassages having gas outlets in an upper surface of the support pins suchthat the wafer floats on gas cushions formed by gas flowing out of thegas outlets in the upper surfaces of the support pins, (d) centering thewafer by moving the wafer across the support pins while the wafer floatson the gas cushions, (e) applying vacuum to the gas passages such thatthe wafer is vacuum clamped to the support pins.

After centering, the method can further comprise (f) rotating the wafer;(g) prerinsing the wafer using a prerinse liquid comprising deionizedwater; (h) thinning a layer of the prerinse liquid by increasing arotational speed of the wafer; and (i) delivering a stream of liquidetchant into the thinned layer of prerinse liquid near an edge bevelarea of the wafer such that the liquid etchant diffuses through thethinned layer of prerinse liquid and substantially removes unwantedmetal selectively from the edge bevel area.

In an embodiment, the chuck includes six support arms, each of thesupport arms having a gas delivery conduit in fluid communication with arespective one of the gas passages in the support pins, wherein during(c) gas flows out of the gas outlets in the upper surfaces of the sixsupport pins and during (e) vacuum is applied to each of the six gaspassages.

In an embodiment, the support pins are elastomeric suction cups whichhave a tendency to stick to the wafer when the wafer is slid across thesupport pins. This causes the wafer to “stick and slip” which createsparticles. By flowing nitrogen gas out of the suction cups, it ispossible to reduce wear of the suction cups and prevent particlegeneration during centering (or moving) the wafer over the support pins.The flow of nitrogen gas out of the suction cups also eliminates curlingor rolling of the lip seal style support pins.

Preferably, the gas is delivered to the gas passages at a pressure of atleast one psi.

The support arms can include rotatable alignment cams, wherein during(d) the alignment cams are rotated from an outer position away from aperiphery of the wafer to an inner position at which the wafer iscentered. The support arms preferably extend outwardly from a rotatablecenter hub, the rotatable center hub having a gas inlet and a pluralityof gas delivery ports in fluid communication with gas delivery conduitsin the support arms, wherein during (c) gas is supplied to the gas inletand flows out of the gas delivery ports to the gas passages in thesupport pins. To hold the wafer, in (e) a vacuum force is applied to thegas inlet and suction is applied to locations on an underside of thewafer by the support pins. The alignment cams can include upper pivotconnections and lower pivot connections, wherein during (d) thealignment cams are rotated about the upper pivot connections by rodsattached to the lower pivot connections. Preferably, the alignment camsare pivotally attached to the support arms so as to be movable betweenalignment positions at which upper portions of the alignment cams centerthe wafer and non-alignment positions at which the upper portions of thealignment cams are located below the wafer. To dry the wafer, the methodfurther includes (j) rotating the alignment cams to the alignmentpositions and (k) drying the wafer by rotating the wafer at a dryingspeed of at least 750 rpm while applying vacuum to the gas passages suchthat the wafer is vacuum clamped to the support pins.

According to a further embodiment, a chuck useful for supporting a waferduring an edge bevel removal (EBR) process, comprises a rotatable centerhub having a plurality of support arms extending outwardly from therotatable center hub, support pins on ends of the support arms, gaspassages extending through upper surfaces of the support pins, and gasconduits in the support arms, the gas conduits configured to supply gasto the gas passages or apply a vacuum to the gas passages.

The chuck can include six support arms, each of the support arms havinga gas delivery conduit in fluid communication with a respective one ofthe gas passages in the support pins. Preferably, the support pins arerubber cups fitted on metal supports located at ends of the supportarms. The metal supports can have vertically extending bores thereinconnecting the gas conduits in the support arms to the gas passages inthe support pins.

The support arms can include alignment cams which are rotatable from anouter non-alignment position away from a periphery of the wafer to aninner alignment position at which the wafer is centered. To supply gasor apply a vacuum force to the gas outlets in the support pins, therotatable center hub can have a gas inlet and a plurality of gasdelivery ports in fluid communication with the gas delivery conduits inthe support arms. Gas can be supplied to the gas outlets by a source ofpressurized gas connected to the gas inlet and suction can be applied tothe gas outlets by a vacuum source connected to the gas inlet.

The alignment cams can include upper pivot connections and lower pivotconnections, wherein the rotatable cams are rotatable about the upperpivot connections by rods attached to the lower pivot connections. Inone arrangement, the chuck includes six support arms located at radialpositions of 60° 120°, 180°, 240°, 300° and 360°, the support arms at60°, 180° and 300° including the alignment cams and the support arms at120°, 240° and 360° not including the alignment cams. The support armshaving alignment cams can include upper arms with the gas deliveryconduits therein and lower arms having actuating rods therein, theactuating rods attached to the alignment cams such that the alignmentcams are rotated toward the periphery of the wafer when ends of theactuating rods move outward from the center hub and are rotated awayfrom the periphery of the wafer when ends of the actuating rods movetowards the center hub.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a group of modules used to formcopper lines in a Damascene process.

FIG. 2 is a perspective diagram of edge bevel removal (EBR) componentsthat can be used in a post-electrofill module.

FIG. 3 shows details of the support arms, support pins and alignmentcams of a chuck useful for EBR.

FIG. 4 shows details of a support arm of a chuck wherein gas can be usedto float a wafer during centering and vacuum can be applied to the waferduring EBR.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, numerous specific embodiments areset forth in order to provide a thorough understanding of embodimentsdescribed herein. However, as will be apparent to those skilled in theart, the claimed invention may be practiced without these specificdetails or by using alternate elements or processes. In other instanceswell-known processes, procedures and components have not been describedin detail so as not to unnecessarily obscure aspects of the claimedinvention.

A “semiconductor wafer” as referred to herein is a semiconductorsubstrate at any of the various states of manufacture in the productionof integrated circuits. One standard semiconductor wafer described inthis invention is 300 mm in diameter, 0.75 mm in thickness, with anapproximate radius of curvature of about 0.15 millimeters (see SEMISpecification M1.15-0997). Of course, semiconductor wafers of otherdimensions, such as a standard 200 mm diameter silicon wafers, can alsobe used (see SEMI Specification M1-0298). Many process parametersdisclosed herein are dependent on wafer size. For example, rotationalspeeds are specified for 300 mm and are inverse proportional to otherdiameters. Therefore, rotational speed of 400 rpm for a 300 mm waferwill be generally equivalent to 600 rpm for a 200 mm wafer.

In embodiments described below, an improved wafer chuck is describedwhich can provide extended wear capabilities of support pins and reducedparticle generation during wafer processing in a post-electrofillmodule” (PEM) designed to carry out edge bevel removal (EBR) andadditional processes such as pre-rinsing, rinsing, acid washing, anddrying. The post-electrofill module (PEM) can be used to carry outvarious process steps following electro-filling of copper on a wafer viaa Damascene process.

FIG. 1 depicts an electrofill system 107 which includes three separateelectrofill modules 109, 111 and 113. System 107 also includes threeseparate post electrofill modules 115, 117 and 119. Each of these may beemployed to perform each of the following functions: edge bevel removal,backside etching, and acid cleaning of wafers after they have beenelectrofilled by one of modules 109, 111, and 113. System 107 alsoincludes a chemical dilution module 121 and a central electrofill bath123. This is a tank that holds the chemical solution used as theelectroplating bath in the electrofill modules. System 107 also includesa dosing system 133 that stores and delivers chemical additives for theplating bath. The chemical dilution module 121 stores and mixeschemicals to be used as the etchant in the post electrofill modules. Afiltration and pumping unit 137 filters the plating solution for centralbath 123 and pumps it to the electrofill modules. Finally, anelectronics unit 139 provides the electronic and interface controlsrequired to operate system 107. Unit 139 may also provide a power supplyfor the system. In operation, a robot including a robot arm 125 selectswafers such as a wafer 127 from a wafer cassette such as a cassette 129Aor a cassette 129B. Robot arm 125 may be attached to the wafer 127 usinga vacuum attachment.

To ensure that wafer 127 is properly aligned on robot arm 125 forprecision delivery to an electrofill module, robot arm 125 transportswafer 127 to an aligner 131. In a preferred embodiment, aligner 131includes alignment arms against which robot arm 125 pushes wafer 127.When wafer 127 is properly aligned against the alignment arms, the robotarm 125 moves to a preset position with respect to the alignment arms.It then reattaches to wafer 127 and delivers it to one of theelectrofill modules such as electrofill module 109. There, wafer 127 iselectrofilled with copper metal. Electrofill module 109 employselectrolyte from a central bath 123.

After the electrofill operation completes, robot arm 125 removes wafer127 from electrofill module 109 and transports it to one of thepost-electrofill modules such as module 117. There unwanted copper fromcertain locations on the wafer (namely the edge bevel region and thebackside) is etched away by an etchant solution provided by chemicaldilution module 121.

Preferably wafer 127 is precisely aligned within post electrofill module117 without making use of aligner 131. To this end, the post electrofillmodules may be provided with an alignment chuck as referenced elsewhereherein. In alternative embodiment, wafer 127 is separately alignedwithin aligner 131 after electrofill and prior to edge bevel removal inmodule 117.

After processing in post electrofill module 117 is complete, robot arm125 retrieves wafer 127 from the module and returns it to cassette 129A.From there the cassettes can be provided to other systems such as achemical mechanical polishing system for further processing.

FIG. 2 schematically illustrates a post-electrofill module whichincludes a chamber in which a semiconductor wafer 224 rotates. Wafer 224resides on a wafer chuck 226 which imparts rotational motion to wafer224. The chamber is outfitted with a drain and associated drain linewhich allows the various liquid streams provided to chamber to beremoved for waste treatment.

A motor controls the rotation of chuck 226. The motor should be easy tocontrol and should smoothly transition between various rotationalspeeds. It may reside within or without chamber. In some embodiments, toprotect against damage from liquid etchant, the motor resides outside ofthe chamber and is separated therefrom by a seal through which arotating shaft passes. Any wobble in the shaft on rotation should besmall (<0.05 millimeters for example) so that the location of fluidnozzles with respect to the wafer does not vary substantially, nor shakethe wafer from its center while it is not confined by alignment orclamping members. Preferably, motor can rapidly accelerate anddecelerate (in a controlled fashion) chuck 226 and wafer 224 at rotationrates between 0 and about 2000 rpm. The motor speed and other operationsshould be controllable by a computer.

Chuck 226 may be of any suitable design that holds wafer 224 in positionduring various rotational speeds. It may also facilitate alignment ofwafer 224 for the etching process. A few particularly preferred examplesof wafer chucks are described below.

The chamber may be of any suitable design that confines the liquidetchant within its interior and allows delivery of the various fluids towafer 224. It should be constructed of an etchant resistant material andinclude ports and nozzles for the various liquid and gaseous streamsused during etching and cleaning.

The EBR process 200 can be carried out by a post-electrofill module. Theprocess begins with a robot arm placing the wafer on the module chuckfor EBR processing. The wafer is typically aligned and placed on a setof support pins that hold the wafer in place by static friction, evenwhen the wafer is later rotated. After the robot arm retracts, deionizedwater is applied to the front of the wafer and the wafer is spun atabout 200-400 rpm in order to pre-rinse the wafer of any particles andcontaminants left over from previous steps. The deionized water is thenturned off and the wafer is spun up to a speed of between about 350-500rpm, which creates a uniformly thin layer of deionized water (wet-filmstabilization). This wet-film stabilization facilitates an evendistribution of the etchant over the front side of the wafer. At thistime, at the latest, any alignment pins or clamps that were used toprecisely align the wafer in the chuck are retracted from the edge ofthe wafer.

After wet-film stabilization, actual removal of the edge bevel metal isperformed. The EBR etchant is typically applied to the surface of thewafer using a thin nozzle tube, which has a nozzle opening at or nearits end. When dispensing a small amount of fluid onto a surface as such,three flow regimes can generally result. The first regime is edgebeading, where surface tension forces dominate the behavior of thefluid, the second is viscous flow, where viscous forces predominate, andthe third is inertial, where inertial forces predominate and the fluidtends to spray. In a specific example, an EBR dispense arm is positionedover the wafer edge and EBR is performed under the following conditions:a total of about 2 to 4 milliliters etchant is delivered at a rate ofabout 0.25 to 2 milliliters/second (more preferably about 0.5 to 1milliliters/second) for a 200 millimeter wafer.

After the required amount of liquid etchant has been applied to the edgeof the wafer, deionized water is again applied to the front side of thewafer as a post-EBR rinse. This application of deionized water willgenerally continue through the subsequent operations of backside etchingand backside rinsing so as to protect the wafer from any extraneousbackside etchant spray and damage. While the deionized water is applied,the dispense arm moves the etchant nozzle away from the wafer.

At generally about the same time, the backside of the wafer ispre-rinsed with deionized water, which is wet-film stabilized in muchthe same manner that the front side of the wafer was (e.g., the waferrotation speed is held at about 350 to 500 rpm). After the flow ofdeionized water to the wafer backside ends, a backside etch (BSE)operation is performed—generally with the same etchant that was used forthe EBR. In a specific embodiment, a thin jet (initially 0.02 to 0.04inches in diameter) of liquid etchant is aimed at the center of thewafer backside. The etchant is preferably delivered from a tubularnozzle having a diameter of about 0.02 to 0.04 inches and a length of atleast about 5 times the diameter. This etchant then disperses over theentire backside of the wafer. The purpose of the BSE is to remove anyresidual copper that was formed on the backside of the wafer duringformation of the seed layer of PVD copper.

The BSE etchant is typically applied using a spray nozzle. Despitegravity, surface tension generally keeps the etchant in contact with thebottom of the wafer long enough to carry out BSE. Since the chuck armscould interfere with the spraying of etchant on the backside of thewafer, the angle of the spray nozzle may be varied during BSE to ensurethorough application of the etchant. Because the wafer is generally heldup by support pins that impinge on the backside of the wafer, theprocess is generally carried out at two different speeds to ensure thatthe etchant flows adequately over the entire surface. For instance, thewafer may be rotated at about 350 rpm during part of the BSE and thenrotated at 500-700 rpm for the remainder of the BSE. The portions of thebackside blocked by the arms will differ at the two speeds, thusensuring complete coverage. Overall, the BSE process typically takes 1-4seconds and uses 1 to 5 cubic centimeters of the preferred etchantdescribed below to reduce the concentration of copper on the backside toless than 5·10⁻¹⁰ atoms per cm² of substrate.

After BSE, both sides of the wafer (or at least the backside of thewafer) are rinsed with deionized water to rinse any liquid etchant,particles and contaminants remaining from the BSE. Then the flow ofdeionized water to the front side ends and about 2 to 4 milliliters of adilute acid, generally less than about 15% by weight acid, is applied tothe front side of the wafer to remove residual metal oxide and removethe associated discoloration. In a specific embodiment, the acid isapplied at a rate of about 2 cc/sec. After the acid rinse, deionizedwater is once again applied to both sides of the wafer, or at least thefront side, to rinse the acid from the wafer. In a specific embodiment,the deionized water is applied for about 15-30 seconds at about 300-400milliliters/min. Finally the wafer can be spun and blow-dried, asdesired, on both sides with nitrogen. Generally, any drying step iscarried out at about 750-2000 rpm for about 10 to 60 seconds, andnecessitates a clamping for the wafer once it reaches about 750 rpm.Many embodiments for the clamping mechanism are possible, and some ofthese are discussed in more detail below. After this processing in thePEM is completed, a robot arm picks up the wafer and puts it in acassette.

Turning again to FIGS. 1 and 2, some features of the PEM will bedescribed in further detail. First, note that wafer 224 rides on supportpins 305 (located on wafer chuck arms 301) by static friction.Preferably, the support pins 305 are located from about 5 to 20millimeters, more preferably about 5 to 10 millimeters, in from the edgeof wafer 224. The design of the support pins is determined by the needto supply enough friction to (1) keep the wafer from flying off thechuck if it is aligned slightly off center (i.e. when aligned to thetolerance of the specification of the edge bevel removal process), (2)not slip as the wafer is accelerated (at typically a rate of 50 to 300rpm/sec (100 rpm/sec in a specific embodiment)) from rest to the EBRrotation rate, and (3) not shed or generate particles. As the wafer'srotational speed increases, however, it reaches a velocity at which thestatic friction from resting on the pins can no longer constrain it dueto small misalignments and the associated centripetal force. To preventthe wafer from flying off chuck 226 at such velocities, clamping cams307 may be employed. The design of suitable cams is described below. Fornow, simply understand that at defined wafer rotational velocities, theclamping cams rotate into a position that locks wafer 224 in place.

Next note that a dispense-arm 303 functions to hold a dispense nozzle256 and move the nozzle into an accurately controlled location over thewafer 224 during the etching step of the process. The dispense-armdesign is not particularly restrictive. It can move down from above thewafer, in from the side, swing in from the edge, rotate down from above,or any combination of these movements. However, the location of thenozzle is preferably reproducibly accurate to within less than about 0.5mm (more typically less than about 0.2 mm) so that the entire etchedregion is mechanically under control. Any suitable pneumatic actuator,solenoid, motor controlled gear, or servo controlled motor can activatethe arm. The dispense-arm should move the dispense nozzle accurately tothe edge of the wafer and move the nozzle out of the way to allow thewafer to be transferred into and out of the chuck. The materials ofconstruction should be resistant to the particular chemical etchingsolution used. If the preferred etchant disclosed herein is used,certain stainless steels (e.g. 303, 625, 316L etc.), ceramics (Al₂O₃,zirconia), Tantalum, and plastic coated metals (polypropylene,polyethylene, PTFE, PVDF) are good choices because they will resistchemical attack, and have sufficient mechanical strength (without creepor flow) to maintain the necessary stringent mechanical tolerances.Similar design considerations hold for the wafer chuck.

As shown in FIG. 3, chuck 226 includes a rotatable center hub 230 havinga plurality of support arms 301 extending outwardly from the rotatablecenter hub. Support pins 305 are located on ends of the support arms301. In an embodiment described below in connection with FIG. 4, thesupport arm can include a gas conduit in fluid communication with a gaspassage in the support pin 305. The support pin is preferably a rubbercup mounted on a metal support at the end of the support arm 301.

In the FIG. 3 embodiment, the alignment cam 307 is a multi-piececomponent which include a pivotable centering finger 309 which can berotated from an inward position at which the wafer is centered to anoutward position (not shown) away from the outer periphery of the wafer.To prevent the wafer from slipping off of the support pins 305 duringrotation of the chuck 226, at a certain rotational speed, centrifugalforce causes a component of the alignment cam to swing outward and causethe centering finger 309 to pivot to the inward position. During someprocessing steps the alignment finger 309 can cause undesirablesplashing of liquids and lead to non-uniform cleaning of the wafer.

To address this problem, the chuck 226 a shown in FIG. 4 includessupport arms 301 a which have gas conduits 311 therein and support pins305 a include gas passages 313 therein which allow the wafer 224 to befloated on a gas cushion during centering and vacuum clamped during EBRprocessing. The support pins 305 a are preferably rubber cups mounted onmetal supports 315 having bores 317 connecting the fluid conduits 311 tothe gas passages 313.

The FIG. 4 embodiment also includes a modified alignment cam 307 awherein a supplemental support arm 301 b includes an actuating rod 319therein which can be moved outwardly away from the center hub 230 topivot the alignment cam 307 a. As shown in FIG. 4, the alignment cam 307a has an upper pivot connection 321 and a lower pivot connection 323.The outer end of the actuating rod 319 is attached to lower pivotconnection 323 and the alignment cam 307 a is attached to the upperpivot connection 321. Thus, when the actuating rod 319 is moved awayfrom the center hub 230, the upper end of the alignment cam 307 a ismoved inward toward the outer periphery of the wafer 224 to center thewafer 224 or prevent the wafer 224 from slipping off of the support pins305 a during high speed rotation of the chuck 226.

As shown in FIG. 4, the gas conduits 311 in the support arms 301 a cansupply gas to the gas passages 313 to float the wafer 224 above thesupport pins 305 a or apply a vacuum to the gas passages 313 to vacuumclamp the wafer 224 on the support pins 305 a. To float a wafer abovethe support pins 305 a, gas can be supplied to the gas passages 313 at 1psi or higher in a pulse lasting a short duration such as 0.5 to 5seconds, preferably about 1 second. For example, nitrogen can besupplied to the gas passages at 1 to 5 psi, preferably about 2 psi tofloat the wafer during wafer centering.

In a preferred embodiment, the chuck 226 a includes six support arms 301a, each of the support arms having a gas delivery conduit 311 in fluidcommunication with a respective one of the gas passages 313 in thesupport pins 305 a. The support arms 301 b include alignment cams 307 awhich are rotatable from an outer non-alignment position away from aperiphery of the wafer 224 to an inner alignment position at which thewafer 224 is centered, as shown in FIG. 4. To supply gas or apply avacuum force to the gas outlets in the support pins 305 a, the rotatablecenter hub 230 a can have a gas inlet 232 and a plurality of gasdelivery ports 234 in fluid communication with the gas delivery conduits311 in the support arms 301 a. Gas can be supplied to the gas outlets bya source of pressurized gas 236 connected to the gas inlet 232 andsuction can be applied to the gas outlets by a vacuum source 238connected to the gas inlet.

As described above, the alignment cams 307 a can include upper pivotconnections 321 and lower pivot connections 323, wherein the rotatablecams 307 a are rotatable about the upper pivot connections by actuatingrods 319 attached to the lower pivot connections 323. In onearrangement, the chuck 226 a includes six support arms located at radialpositions of 60° 120°, 180°, 240°, 300° and 360°, the support arms atradial positions 60°, 180° and 300° including the alignment cams 307 aand the support arms at radial positions 120°, 240° and 360° notincluding the alignment cams 307 a. The support arms having alignmentcams 307 a can include upper arms 301 a with the gas delivery conduits311 therein and lower arms 301 b having actuating rods 319 therein, theactuating rods 319 attached to the alignment cams 307 a such that upperends of the alignment cams 307 a are rotated toward the periphery of thewafer 224 when ends of the actuating rods 319 move outward from thecenter hub 230 a and are rotated away from the periphery of the wafer224 when ends of the actuating rods 319 move towards the center hub 230a. During centering of a wafer, a robot drops the wafer onto the supportpins 305 a and while the wafer 224 is supported on the pins 305 a, gasis supplied to the gas passages 313 at a pressure sufficient to floatthe wafer above the support pins 305 a and the alignment cams 307 a arerotated from outer positions to inner positions at which one or morealignment cams 307 a contact the periphery of the wafer and move thewafer to a position at which the center of the wafer aligns with thecenter axis of the chuck 226 a.

A typical Damascene process begins with formation of line paths in apreviously formed dielectric layer, which may be etched with trenchesand vias. The lines define conductive routes between various devices ona semiconductor wafer to be filed with conductive materials. The processcontinues with depositing a thin diffusion barrier layer to preventdiffusion of the conductive materials into the dielectric layer.Suitable materials for the diffusion barrier layer include tantalum,tantalum nitride, tungsten, titanium, and titanium tungsten. In atypical embodiment, the barrier layer is formed by a PVD process such assputtering. The next following operation involves depositing aconductive seed layer to provide a uniform conductive surface forcurrent passage during an electrofill operation. A PVD method may beemployed for this operation. The wafer is then electrofilled with athicker layer of copper over the seed layer. Electrofilling continuesuntil the line paths completely filled to the top surface of thedielectric.

It is desirable to use as much of the wafer surface for active circuitryas possible. While it is generally possible to provide some shieldingduring electroplating, similar shielding is not as straightforward forPVD. Therefore, during the PVD seed layer formation copper is depositedin some unwanted areas, such as bevel edge region. Thick copperdeposition may result in higher currents in this area during theelectrofill adding even more metal into the undesirable areas forming abevel-like shape on the edge of the wafer. This bevel may easily breakaway during later CMP and damage devices on the surface of the wafer. Asa result, the bevel must be removed, which is accomplished by the EBRand/or backside etch (BSE) processes.

With EBR, etchant is applied to the front edge of the wafer in a thinstream. In certain embodiments, the etchant is applied under viscousflow conditions to remain thin over the thinned layer of the pre-rinseliquid. The etchant is generally applied with some radial velocitycorresponding to the flow rate and nozzle orientation. Additionally, theetchant is forced to the edge of the wafer by the centrifugal forceresulting from the rotation of the wafer. The combination of these twoforces with gravitational force and surface tension, causes the etchantto flow outward, and down over the side edge and onto a few millimetersonto the backside, thus accomplishing removal of the unwanted metal fromall three of these areas. After EBR, the electroplated copper isplanarized, generally by CMP down to the dielectric in preparation forfurther processing, generally the addition of subsequent dielectric andmetalization layers.

EBR Process Details

The wafer begins to spin at, e.g., about 150-400 rpm and deionized wateris applied to the front of the wafer. Wafer rotation serves to evenlydistribute the applied deionized water over the wafer surface and toremove the excess of water from the wafer from surface. This pre-rinsingremoves particles and contaminants left over from previous processingsteps. Moreover, pre-rinse wets the front side of the wafer that may bedry after the previous processing steps. In one embodiment, thepre-rinse operation employs only deionized water and no acid. Thepre-rinse operation takes place anywhere between 1 to 5 seconds with aflow rate of 200-800 ml/minute depending on rinse water temperature,plating chemistry, deionized water flow rate and the rotational speed ofthe wafer. It is sometime desirable to use hot rinse water to acceleratethe pre-rinse efficiency. Therefore, DI water at from 20 to 50° C. canbe employed depending on the economics of the operations.

Creating a uniform water film on the wafer surface is frequentlydesirable. Using a “clamshell” or other wafer clamping tool thatexcludes the wafer edge during plating often results in parts of waferedges being dry, while other being wet. An etching process may beineffective and even damaging to the wafer if etchant is distributedover an unevenly wetted edge.

It may be desirable to have a uniform but thin layer of water in theareas where etchant is applied. A thinner film provides for fasterdiffusion of the etchant to the metal and smaller taper width on theetched edge of the metal. To produce a thinner film, the deionized wateris turned off after pre-rinse operation, and the wafer rotational speedis increased substantially (e.g., to about 400-1300 rpm in certainembodiments) for approximately a relatively short duration (e.g., about1 to 5 seconds in certain embodiments) allowing for wet film thinning.In a specific embodiment, the wafer is rotated at about 600-1200 rpm forapproximately 1.5-3 seconds. These parameters may depend on the wafersize, surface tension of the pre-rinse liquid that may be modified withvarious surfactants, and other factors. Higher rotation speed leads togreater centrifugal force experienced by the layer of the pre-rinseliquid. This force is directed away from the center of the wafer andtherefore removes some of the pre-rinsed liquid from the wafer.Moreover, higher centrifugal forces may provide better uniformity of thelayer. Further, the higher rotational speeds of the wet film thinningoperation enhance evaporation of the pre-rinse liquid from the surfaceof the wafer further thinning the layer of the remaining liquid.However, rotation speeds should not exceed levels at which thin waterlayer loses its uniformity, i.e. breaks apart, or wafer loses itsalignment.

The wet film thinning operation may include delivering of thinning fluidto the edge area or to the entire surface of the wafer. Thinning fluidsmay reduce the surface tension and increase vapor pressure of theresulting solution. Lowered surface tension changes the contact angle ofthe layer at the edge bevel area leading to a smaller bead. At the sametime, higher vapor pressures increase evaporation. For example, a highvapor pressure organic solvent, like isopropyl alcohol (“IPA”), may bedelivered on the top of the pre-rinsed liquid through a nozzle similarto delivering of the etchant during the EBR operation.

The thinning fluid may be also pre-heated and be applied together withother liquids or carrier gases to further heat the edge liquid and thindown the liquid layer further reducing the surface tension andviscosity. The thinning fluids may be delivered right after thepre-rinse operation and before the wafer is accelerated. In anotherembodiment, thinning fluids may be applied during or after accelerationof the wafer.

The wet film thinning operation may include use of an impinging flowinggas to help physically remove excess fluid from the periphery. In suchoperation, a directed jet of gas flowing through a nozzle near the waferperiphery imparts momentum and an added force on the liquid bed, forcingentrained fluid outward and away from the bevel and allowing it to bequickly thinned.

In an alternative approach typically requiring a lower velocity and rateof flowing gas, an edge liquid bead thinning technique imparts aliquid-surface-tension-reducing gas stream, typically an organiccompound in gas, vapor, or aerosol form, blown through a nozzle onto thesurface of the liquid edge bead layer. Molecules of thesurface-tension-lowering stream are adsorbed into the liquid layer onthe wafer surface. By passing a soluble and surface tension loweringadsorbate over the air-liquid interface, the air-liquid interfacialtension of the fluid adhering to a wafer is reduced, thereby alteringthe balance of forces between centrifugal and surface tension forces andallowing the bead to be thinned. Suitable surface tension reducingchemicals tend to be volatile, soluble in the water, and have somespatially separated polar and non-polar molecular groups so that, likemost surfactants, they can align non-polar groups to the surface andpolar groups with the internal regions of the fluid, thereby reducingsurface energies and forces. Isopropyl alcohol (IPA) is one typicalexample of a chemical used in a vapor or an aerosol form to achieve thisliquid-layer thinning result. Other examples include other alcohols(ethanol, butanol), amines (ethyl and propylamine), ketones (MEK) andaldehydes (acetylaldehyde) that have hydrophilic and hydrophobicmolecular groups. From a different perspective, the thinning liquid maybe chosen to significantly increase the vapor pressure of the prerinseliquid. In one embodiment, ultrasonic oscillation operating at 25-120kHz may be used to create IPA aerosol and aid in the rapid diffusion ofgas into the edge bead fluid. Typically, IPA is fed at about 1 ml/min to100 ml/min at a mole fraction of 2 to 30% in a carrying gas (e.g.nitrogen), depending on the configuration of the nozzle and otherprocess parameters. Other vapors and aerosols may also be used andcombination of physical (force of flowing gas) and chemical (reducedsurface tension) removal of the bead are also possible.

The film thinning operation produces a generally uniform thin aqueouslayer. The etchant is then delivered, as depicted in an EBR operation,on the top of this aqueous layer in the edge areas of the wafer anddiffuses through the layer to contact the metal. A thinner pre-rinselayer provide for faster etchant diffusion and less dilution. Moreover,the etchant is localized in the edge area rather than diffusing throughthe aqueous layer towards the center of the wafer leading to increasedtaper width.

The process continues with an edge bevel removal (EBR) operation. Incertain embodiments, the wafer is rotated at about 150-400 rpm, morepreferably about 200 to 250 rpm for 200 mm wafers and about 175 to 225rpm for 300 mm wafers. This rotational speed ensures coverage of theentire edge area with the EBR etchant. The acceleration of the waferduring the wet film thinning operation and deceleration during the EBRoperation may be performed at rates that ensure continued alignment ofthe wafer in the chuck. In certain embodiments, the rotational rate doesnot exceed about 150 rpm/sec when using typical plastic support pins(e.g., PPS or PVDF). Pins with greater friction coefficient may be usedas long as they do not flake or generate particles.

The EBR etchant and the edge bead liquid surface tension reducing streamare typically applied to the surface of the wafer using a thin tube witha nozzle opening at or near its end. When dispensing a small amount ofetchant onto a surface as such, three flow regimes can generally result,any of which may be appropriate. The first regime is edge beading, wheresurface tension forces dominate the behavior of the fluid, the second isviscous flow, where viscous forces predominate, and the third isinertial, where inertial forces predominate and the fluid tends tospray. The EBR operation can be performed under the followingconditions: a total of about 2 to 14 milliliters etchant is delivered ata rate of about 0.25 to 2 milliliters/second (more preferably about 0.3to 0.5 milliliters/second). The amount delivered depends on the filmthickness to be removed, size of the wafer, the concentration ofchemical etchant, rotation rate and etchant temperature.

The etchant can be delivered in several stages. For example, the etchantmay be delivered in two stages: a high flow rate stage followed by alower flow rate stage. During the high flow rates stage the etchant maybe delivered at about 0.25-0.35 ml/s for about 1-5 seconds followed bythe low flow rate stage with delivery rate of about 0.10-0.20 ml/s forabout 10-30 sec. The high flow rate stage helps the EBR etchant overcomethe surface tension resistance of the pre-rinse film and rapidly diffusethrough the layer. At this stage, diffusion of the etchant within thefilm is facilitated because the film is originally free from etchant.However, the duration of this stage should not exceed the time requiredfor the etchant to saturate the film. The low flow rate stage thensupplies the bulk of the etchant for EBR. The flow rate should be smallenough to prevent excessive diffusion of the etchant into the activepart of the wafer, which may result in the wider taper. The deliveryrate and the duration at this stage may depend on a wafer diameter(length of the edge bevel), bevel thickness, pre-rinse film thickness,and other factors. Excessive etchant may result in wider taper width. Ina specific embodiment optimized for up to about 0.75 micrometer thickbevels on a 300-mm wafer, about 2-4 ml of etchant is delivered over aperiod of approximately 15-20 seconds.

The etchant may include an acid and oxidizer. Examples of acids that areuseful include sulfuric acid, hydrohalic acids, chromic acid and nitricacid. In one embodiment, the etchant for copper EBR may be a solution ofH₂SO₄ (sulfuric acid) and H₂O₂ (hydrogen peroxide) in water. In onespecific embodiment, the etchant comprises between about 15% to 25%H₂SO₄ by weight and 20% to 35% H₂O₂ by weight. A thinner film of thepre-rinsed liquid may allow higher acid concentration in the etchant.Other oxidants, such as peroxydisulfate S₂O₈ ⁻² and concentrated HNO₃(about 30% in water), may be used. Near neutral and alkaline etchantswhich tend to complex with the dissolved metal can also be employed,such as combinations of glycine or ethylene diamine and hydrogenperoxide at a pH of around 9. Generally, the liquid etchant should havephysical properties compatible with the etching system, such as surfacetension, contact angle, and viscosity.

After the required amount of liquid etchant has been applied to the edgeof the wafer, deionized water may be applied to the front side of thewafer as a post-EBR rinse. Deionized water may be applied to the entirewafer as a whole and not just the wafer edge. This application ofdeionized water will generally continue through the subsequentoperations of backside etching and backside rinsing so as to protect thewafer from any extraneous backside etchant spray and damage. While thedeionized water is applied, the dispense arm moves the etchant nozzleaway from the wafer.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the present invention. Accordingly, the presentembodiments are to be considered as illustrative and not restrictive,and the invention is not to be limited to the details given herein. Allreferences cited herein are incorporated by reference for all purposes.

What is claimed is:
 1. A method of centering a semiconductor wafer priorto removing unwanted metal during an edge bevel removal (EBR) process,the method comprising: (a) transferring a wafer above a rotatable chuckhaving at least three support arms with support pins at outer portionsof the support arms; (b) lowering the wafer onto the support pins; (c)supplying pressurized gas to gas passages having gas outlets in an uppersurface of the support pins such that the wafer floats on gas cushionsformed by gas flowing out of the gas outlets in the upper surfaces ofthe support pins; (d) centering the wafer by moving the wafer across thesupport pins while the wafer floats on the gas cushions; and (e)applying vacuum to the gas passages such that the wafer is vacuumclamped to the support pins.
 2. The method of claim 1, furthercomprising: (f) rotating the wafer; (g) pre-rinsing the wafer using apre-rinse liquid comprising deionized water; (h) thinning a layer of thepre-rinse liquid by increasing a rotational speed of the wafer; and (i)delivering a stream of liquid etchant into the thinned layer ofpre-rinse liquid near an edge bevel area of the wafer such that theliquid etchant diffuses through the thinned layer of pre-rinse liquidand substantially removes unwanted metal selectively from the edge bevelarea.
 3. The method of claim 1, wherein the chuck includes six supportarms, each of the support arms having a gas delivery conduit in fluidcommunication with a respective one of the gas passages in the supportpins, wherein during operation (c) gas flows out of the gas outlets inthe upper surfaces of the six support pins.
 4. The method of claim 1,wherein the chuck includes six support arms, each of the support armshaving a gas delivery conduit in fluid communication with a respectiveone of the gas passages in the support pins, wherein during operation(e) vacuum is applied to each of the six gas passages.
 5. The method ofclaim 1, wherein the gas is delivered to the gas passages at a pressureof at least one psi.
 6. The method of claim 1, wherein the support armsinclude rotatable alignment cams, wherein during operation (d) thealignment cams are rotated from an outer position away from a peripheryof the wafer to an inner position at which the wafer is centered and gasis supplied to the gas passages in a pulse as the alignment camsapproach the outer periphery of the wafer.
 7. The method of claim 1,wherein the support arms extend outwardly from a rotatable center hub,the rotatable center hub having a gas inlet and a plurality of gasdelivery ports in fluid communication with gas delivery conduits in thesupport arms, wherein during operation (c) gas is supplied to the gasinlet and flows out of the gas delivery ports to the gas passages in thesupport pins.
 8. The method of claim 7, wherein during operation (e) avacuum force is applied to the gas inlet and suction is applied tolocations on an underside of the wafer by the support pins.
 9. Themethod of claim 6, wherein the alignment cams include upper pivotconnections and lower pivot connections, wherein during operation (d)the alignment cams are rotated about the upper pivot connections by rodsattached to the lower pivot connections.
 10. The method of claim 2,wherein alignment cams are pivotally attached to the support arms so asto be movable between alignment positions at which upper portions of thealignment cams center the wafer and non-alignment positions at which theupper portions of the alignment cams are located below the wafer, themethod further comprising: (j) rotating the alignment cams to thealignment positions; and (k) drying the wafer by rotating the wafer at adrying speed of at least 750 rpm while applying vacuum to the gaspassages such that the wafer is vacuum clamped to the support pins. 11.A method of centering a semiconductor wafer, the method comprising:supporting a wafer above a rotatable chuck, the chuck including at leastthree support arms and an array of support pins and gas outlets, eachsupport arm including a respective support pin disposed at an outerportion of a respective support arm, each support pin including one ofthe gas outlets disposed in an upper surface thereof; lowering the waferonto the array of support pins; supplying pressurized gas to the gasoutlets such that the wafer floats on gas cushions formed by gas flowingout of the gas outlets; centering the wafer by moving the wafer acrossthe support pins while the wafer floats on the gas cushions; andapplying vacuum to the gas outlets such that the wafer is vacuum clampedto the support pins.
 12. The method of claim 11, further comprising:rotating the wafer; pre-rinsing the wafer with a pre-rinse liquid;thinning a layer of the pre-rinse liquid by increasing a rotationalspeed of the wafer; and delivering a stream of liquid etchant into thethinned layer of pre-rinse liquid near an edge bevel area of the wafersuch that the liquid etchant diffuses through the thinned layer ofpre-rinse liquid and substantially removes unwanted metal selectivelyfrom the edge bevel area.
 13. The method of claim 11, wherein the chuckincludes six support arms, each of the support arms including a gasdelivery conduit in fluid communication with a respective one of the gasoutlets, and wherein the gas cushions are formed by gas flowing out ofthe gas outlets in the upper surfaces of the support pins.
 14. Themethod of claim 11, wherein the chuck includes six support arms, each ofthe support arms including a gas delivery conduit in fluid communicationwith a respective one of the gas outlets in the support pins, the methodfurther comprising applying vacuum to each of the gas outlets.
 15. Themethod of claim 14, wherein the gas is delivered to a gas deliveryconduit at a pressure of at least one psi.
 16. The method of claim 14,wherein the support arms include rotatable alignment cams, and whereinthe method further includes: rotating the alignment cams from an outerposition away from an outer periphery of the wafer to an inner positionat which the wafer is centered; and supplying gas to the gas deliveryconduit in a pulse as the rotatable alignment cams approach the outerperiphery of the wafer.
 17. The method of claim 16, wherein thealignment cams include upper pivot connections and lower pivotconnections, and wherein the method further comprises rotating thealignment cams about the upper pivot connections by rods attached to thelower pivot connections.
 18. The method of claim 11, further comprising:providing alignment cams pivotally attached to the support arms so as tobe movable between alignment positions at which upper portions of thealignment cams center the wafer and non-alignment positions at which theupper portions of the alignment cams are located below the wafer;rotating the alignment cams to the alignment positions; and drying thewafer by rotating the wafer at a drying speed of at least 750 rpm whileapplying vacuum to the gas passages such that the wafer is vacuumclamped to the support pins.
 19. The method of claim 11, wherein thesupport arms extend outwardly from a rotatable center hub, the rotatablecenter hub having a gas inlet and a plurality of gas delivery ports influid communication with gas delivery conduits provided in the supportarms, and wherein the method further comprises supplying gas to the gasinlet for delivery to the gas outlets in the support pins via theplurality of gas delivery ports of the rotatable center hub.
 20. Themethod of claim 19, further comprising applying a vacuum to the gasinlet of the center hub such that a suction is applied to locations onan underside of the wafer by the gas outlets of the support pins.